A fractionation tower is an efficient unit widely used in the petroleum and chemical industries for separating the constituent components of a mixture of two or more materials having different boiling points. Frequently, it is used to separate hydrocarbons having different compositions. In this tower, feed is heated causing the vapors to rise and the liquid to descend. As the hot rising vapors contact the cooler descending liquid, a heat interchange takes place. The vapors are cooled and some of the higher boiler constituents condense. The heat of condensation given up to the descending liquid vaporizes some of its lower boiling constituents. Hence, there is a gradual enrichment of the lower boiling constituents in the vapors as they rise in the tower, and an enrichment of the higher boiling constituent in the liquid as it descends.
The operation of the tower depends on intimate contact of the distilling vapors and the descending liquid. To achieve this intimate contact, the tower usually includes a series of horizontal trays stacked one on top of another. These trays commonly have perforated bottoms which permit the liquid to flow downwardly as the vapors rise through liquid collected in the trays. In most commercial units, the mixture of materials is usually preheated and flows into the tower at about the tower midpoint. Heat is usually supplied by withdrawing some of the material from the tower bottom, recycling it through a heat exchanger to the tower bottom. with steam being introduced into the heat exchanger to elevate the temperature of the withdrawn and recycled material. Eventually, the lower boiling material leaves the top of the tower and flows into a heat exchanger or condenser, and is condensed and collected in a receiving drum. A portion of this condensed material is withdrawn from the drum as distillate and another portion is recycled as reflux to the top of the tower. Because cooler reflux is added to the top of the tower, more heat must be introduced into the bottom of the tower to revaporize or "reboil" the reflux. Thus, any heavy material which may reach the top of the tower as vapor is condensed and rinsed back down the tower by the reflux. Any light material which may work its way into the bottom of the tower is revaporized or reboiled by the extra heat.
Transient variations in feed rate and feed composition will cause undesirable variations in fractionation tower pressure. These variations in tower pressure will in turn cause either heavier vapor to rise in the tower or lighter liquid to "spill" down the tower detracting from the "fractionation efficiency" of the tower. This results in loss of product and damaged specifications. It can be partially corrected by additional heat and reflux, but this is costly in utilities consumption.
It is then desirable to seek a method which will accurately control pressure without excess cost in use of heat or reflux. It is most desirable to achieve a method of pressure control which allows almost no deviation even under conditions of radically changing rates of feed to the tower or under radically changing composition of that feed.
Many distillation columns encounter continuously changing feed rate and/or changing feed composition such that they remain in a constant state of upset. Such columns cannot be operated near capacity or at optimum utility consumption and seldom achieve their design fractionation efficiency. This is in large part due to the inability of the control system to anticipate changes in heat input demand either soon enough or in the proper quantity to avoid upsets resulting from the considerable time lags involved.
The prior art is filled with a variety of systems such as U.S. Pat. No. 3,411,308; U.S. Pat. No. 3,449,215; U.S. Pat. No. 3,840,437; U.S. Pat No. 3,428,528; and U.S. Pat. No. 3,415,720 which are used to control the heat input to a fractionation tower. U.S. Pat. No. 3,411,308 teaches that the flow rate of a bottoms product stream from a fractionator is regulated responsive to the difference between a delayed function of the flow rate of the feed and the sum of the flow rates of the remaining product streams. The overhead product flow rate signal is a bias value of the output of liquid level controller on the overhead accumulator instead of an actual flow measurement. The flow rate of a medium purity side draw stream is manipulated responsive to the difference between a signal representing a computed yield of the component of interest and a signal representing measured yield. U.S. Pat. No. 3,449,215 discloses that in a fractionation process in which a product flow rate is predicted from feed analysis and other factors, a signal representative of the computed product flow rate is used to control the product flow rate, and an analyzer determines the concentration of a key component in a product stream and accordingly provides a biasing signal for the computed product flow rate signal as a feedback correction thereto, an additional correction to the feedback signal is made to increase the stability of the system and to prevent conditions such as oscillation in the system which can be caused by an uncorrected feedback signal when operating or feed conditions vary. U.S. Pat. No. 3,840,437 teaches a multi-component feed stream is separated in a distillation column to provide an overhead distillate product stream and a bottoms product stream. The bottoms product stream flow rate is controlled at a level substantially equal to the computed flow rate based on a material balance determined from measurements on feed flow rate, feed composition and product specifications thereby producing a bottoms product having the desired composition. A signal representative of the predicted internal reflux flow rate is established based on measurements of feed flow rate, feed composition, feed enthalpy and product specifications. The actual internal reflux flow rate is determined and compared with the predicted internal reflux flow rate to obtain a bias signal. The bias signal is used to control the external reflux flow rate at a level such that an overhead distillate product having the desired composition is obtained. U.S. Pat. No. 3,428,528 teaches that in a fractionation system having an overhead product, a sidedraw product and a bottom product, the flow rate of each of the external reflux, the sidedraw product stream and the bottom product stream is manipulated by a respective output of a computer control system. The input of the computer control system include the feed flow rate, feed component concentrations, and desired component concentrations in each of the three product streams. The computer control system performs the simultaneous solution of three interrelated predictive equations to obtain the three flow control signals. And U.S. Pat. No. 3,415,720 relates to a method and apparatus for automatically controlling a continuous distillation process for the separation of a feed stream into a top and a bottom product stream in a column where reflux and reevaporation are applied and wherein the quality of the two product streams is controlled. However, none of these systems are able to achieve the improved results of our invention. Our invention can be differentiated from the prior art in that our invention incorporates a simple yet unique relationship which provides the correct anticipation of heat demand. This provides sustained product specification and proper and economical operating conditions even during radical changes of feed rate or feed composition. Furthermore, our invention properly controls heat input in a distillation column even during conditions of severe and extreme operating conditions, providing stability when prior control devices permit unstable operation. The results include improved product quality control, utilities saving and increased capability of the distillation column, particularly when continuously changing conditions exist.